Creation of arbitrary coherent superposition states in four-level systems

Using the technique of stimulated Raman adiabatic passage, we propose schemes for creating arbi- trary coherent superposition states of atoms in four-level systems: a A-type system with twofold final states and a four-level ladder system. With the use of a control field, arbitrary coherent superposition states are created without the condition of multiphoton resonance. Suitable manipulation of detunings and the control field can create either a single state or any superposition states desired. (c) 2005 Pleiades Publishing, Inc.

Mechanisms of femtosecond laser-induced breakdown and damage in MgO

Single-shot laser damage threshold of MgO for 40-986 fs, 800 nm laser pulses is reported. The pump-probe measurements with femtosecond pulses were carried out to investigate the time-resolved electronic excitation processes. A theoretical model including conduction band electrons (CBE) production and laser energy deposition was applied to discuss the roles of multiphoton ionization (MPI) and avalanche ionization in femtosecond laser-induced dielectric breakdown. The results indicate that avalanche ionization plays the dominant role in the femtosecond laser-induced breakdown in MgO near the damage threshold. (c) 2005 Elsevier B.V. All rights reserved.

飞秒双脉冲激光在硅和石英中的光学破坏

The optical breakdown thresholds (OBTs) of typical dielectric and semiconductor materials are measured using double 40-fs laser pulses. By measuring the OBTs with different laser energy and different time delays between the two pulses, we found that the total energy of breakdown decrease for silica and increase for silicon with the increase of the first pulse energy. (C) 2005 Optical Society of America.

周期量级超短激光脉冲作用下导带电子的光吸收与碰撞电离

激光照射下光学材料的损伤过程中，导带电子的加热和碰撞电离是非常重要的过程，影响着导带电子的产生、晶格能量的沉积和破坏．分析了Drude模型的局限性，从经典力学出发求解了周期量级激光场中导带电子的运动方程，计算了导带电子的光吸收和碰撞电离，分析了激光强度、载波相位等对碰撞电离的影响．

飞秒强激光场中大尺寸氩团簇爆炸机理的实验研究

By using time-of-flight spectroscopy, the ionization and explosion of large argon clusters ( (n) over bar = 3 x 10(3) - 3 x 10(6)) in the intense femtosecond pulsed laser field (60 fs,2 x 10(16) W/cm(2)) has been studied, and the dependence of average energy of ions emitted from argon clusters on the gas backing pressure has been measured. By comparing the average ion energies obtained with two different supersonic conical nozzles and considering the Hagena's scaling law of clusters, we have found that the average ion energy is determined by the cluster size when the laser parameters are kept unchanged. The experimental results indicated that when the cluster size is less than 3 x 10(5) atoms per cluster, the Coulomb repulsion force is the dominating factor in the expansion mechanism. Beyond this size, for 3 x 10(5) < (n) over barn < 3 x 10(6), the expansion is the result of the combined effect of both the Coulomb repulsion force and the hydrodynamic force, and the latter will play the dominating role for increasing cluster size.

飞秒激光脉冲在正色散固体材料中的自压缩

实验研究了正色散固体介质中的激光脉冲自压缩现象，证明了无需任何外加色散补偿情况下，固体透明介质中的自聚焦传输过程可使高功率飞秒激光脉冲实现时域脉冲压缩，并详细研究了输出脉冲的时域和频域特性随入射脉冲强度的演化规律．实验结果表明脉冲自压缩量随入射脉冲强度的增加呈递增趋势，然而当入射光强增大到足以引起超连续谱及锥形辐射产生时，脉冲时域形状会发生分裂．此外还发现发散光束入射情况下同样可以观察到脉冲自压缩现象．

飞秒激光对氟化镁烧蚀机理研究

用扫描电镜（SEM）研究了氟化镁在800nm超短脉冲激光作用下的单枪表面烧蚀形貌．根据烧蚀斑面积与激光脉冲能量间的对数关系，测得烧蚀阈值与激光脉宽的关系曲线（55—750fs）．计算了导带电子的双光子吸收，改进了多速率方程模型．很好地解释了实验结果．

Compact 120 TW Ti: sapphire laser system with a high gain final amplifier

A 120TW/36fs laser system based on Ti:sapphire chirped-pulse amplification (CPA) has been successfully established in our lab. The final four pass Ti:sapphire amplifier pumped by an energetic single-shot Nd:YAG-Nd:glass laser was designed and optimized. With 24J/8ns pump energy at 532 nm, 300 mJ/220 ps chirped pulse was amplified to 5.98 J in this amplifier, and a total saturated gain of similar to 20 was achieved. The focused intensity of compressed beam could reach to 10(20) W/cm(2) with the M-2 of similar to 2.0. (c) 2005 Elsevier Ltd. All rights reserved.

利用整形激光脉冲压缩薄膜靶得到中子源

研究了整形激光脉冲对冰冻氘氚靶的压缩。通过数值分析，发现利用分步激光产生的系列激波压缩氘氚靶可以获得较高密度和较低温度的等离子体靶。初始激光强度的选取将影响到压缩后的等离子体密度，继而影响到产生中子的数量。通过调节初始激光强度可以使压缩后的氘氚靶温度处在反应率比较高的范围内，从而得到优化结果。当初始归一化激光振幅为0．5，最终为32时，压缩后的氘氚靶密度可达到18416倍的临界密度，温度达到16keV，每焦耳入射激光能量可得到10。个中子，这个中子产额比现有其他方法所得到的中子产额大4个数量级。

强激光在窄通道中传播的理论研究

在考虑相对论和有质动力非线性以及全局电量守衡的前提下，分析了强激光在冷等离子体窄通道中稳定传播的情况。采用较为简化的二维理论模型，给出了描述激光和通道横向结构的解，对不同通道宽度、通道密度、激光强度和电磁模式等进行了讨论，分析了其对激光在等离子体通道中传播的影响。分析发现，在存在预通道的情况下，当等离子体通道的密度大于临界密度很多时（例如20倍临界密度），即使是在激光波长量缴的通道中，激光仍然可以传播。通道越宽，等离子体密度越小；激光强度越大越容易传播。在同样的通道和传输情况下，TE0模传输所需要的激光强